Every smudge of light in this image represents a distant galaxy. The depth of field is infinity. The more complex spiral structures visible at closer range are cousins to the Milky Way, our home galaxy.

This view of ultra-deep space is part of a larger photograph assembled from 800 exposures taken by the Hubble Space Telescope between 2003 and 2004. The image frames a region of starless space within the southern constellation Fornax. The dark void in Fornax is like a hole in the galactic envelope through which we can glimpse extremely remote phenomena.

The larger image, known as the Hubble Ultra Deep Field (download at NASA's Web site), encompasses an area of the sky just one-tenth as wide as the apparent size of the full moon. Yet it is estimated to contain 10,000 galaxies extending over billions of light years. The low-resolution view on this page represents only one-quarter of the larger image -- perhaps about 2,500 galaxies.

Although this ultra-deep view might suggest that galaxies are distributed randomly through space, astronomical observations reveal otherwise. Many (if not most) galaxies are gravitationally bound to their near neighbors in clusters and superclusters. The Milky Way belongs to a cluster of more than 30 galaxies known as the Local Group, of which it is the second-largest member.

Recent research indicates that the visible matter of the universe is embedded in a framework or scaffolding of so-called dark matter, a phenomenon that remains poorly understood. Dark matter pervades the universe in a network of invisible filaments characterized by clumps and voids. Galaxies coalesce in regions of densely intertwined filaments, which constrain their movements and their interactions.